Evergreen has featured in many descriptions of Ryan Giggs since he landed the Professional Footballers' Association's player of the year award on Sunday night but the inaccuracy of the tribute must torment Sir Alex Ferguson. Not only is time marching against the great Welshman, he is also a glaring illustration that a problematic succession at Manchester United is not confined to the manager's chair.

While 35-year-old Giggs accepted the votes and applause from fellow professionals to take the top award for the first time in 18 seasons, this weekend brought a new low in the Old Trafford career of his heir apparent, the Portugal international Nani. Hauled off at half-time by Ferguson with United trailing 2-0 to Tottenham Hotspur, the 22-year-old responded in a way that confirmed to his many doubters inside Old Trafford that he is not fit to succeed Giggs on or off the pitch; he left the stadium before United had completed their latest stirring recovery and kept their noses in front in the race for the Premier League title.

United say that Nani did not break any club rules by making a hasty exit and no disciplinary action is therefore planned. The player's representatives claimed he was "very upset" at being replaced and received permission from the club to go home. To those United supporters who witnessed the winger outside the stadium on Saturday, however, and the many who posted on internet message boards yesterday, it was a lack of professionalism that should hasten his exit permanently in the summer.

Bought for £18m from Sporting Lisbon in 2007, Nani, according to Ferguson, would be groomed to replace Giggs on United's left wing while Anderson, the £17m signing from Porto on the same day, would follow suit for Paul Scholes. Almost two years later the search for Giggs' successor continues – United have since also acquired Zoran Tosic from Partizan Belgrade – while Nani's development appears to have gone into reverse.

Saturday was only the sixth time this season that he has started a league game for the champions. Not once has he lasted longer than 70 minutes in those six. In fairness to the winger he has scored six goals during this campaign, although even that roll-call illuminates where he resides in the order of selection at Old Trafford, the opponents being Middlesbrough, West Bromwich Albion, Blackburn Rovers, Southampton and Derby County. The Portugal international remains capable of moments of individual brilliance, but it is his lack of game intelligence that has left a greater impression on an Old Trafford crowd now exasperated with his anonymous talent. In stark contrast to compatriot and fellow Sporting Lisbon product, Cristiano Ronaldo, there has been scant improvement in Nani during his two seasons with United.

Nani claimed that his grievance at Old Trafford on Saturday stemmed from his own performance level. "I was not satisfied with my work against Tottenham but I accept the orders of the coach with the half-time substitution," he said last night. "In the second half the team changed in every respect but I alone am not responsible for the bad performance in the first 45 minutes. I finished the game unhappy with my work but I still believe I can demonstrate my level at United."

Whether he will get that opportunity, or can take it, is in serious doubt. Ferguson has been tolerant with Nani to some extent, giving up on his attempts to curb the winger's somersault goal celebrations, for example, and dismissing the head-butt on Lucas Neill last season that brought him a three-match ban as a sign of "immaturity". But the manager must be acutely aware that patience with the Portuguese is fading at Old Trafford.

Nani is, unsurprisingly, close to Ronaldo and shares the same agent, Jorge Mendes. It is suspected the 22-year-old may follow Ronaldo out of Old Trafford at the end of this season should the world player of the year get his wish to join Real Madrid, with Italy and Internazionale, coached by another Mendes client, Jose Mourinho, a possible destination.

"My head is in Manchester United and the rumours about other clubs are not true," added Nani. "I am at the best club in the world and it is my desire to stay here. My dream is to secure a place in the first team. I have not shown my best this season because of injuries but I am a professional and I will fight for this club. I do not have any problems with the coach. I will always be grateful to the manager for this opportunity and I am still learning. I will never reject the challenge to be a star for this team." A genuine star, Giggs, meanwhile, looks ahead to Arsenal and his 800th appearance for United.